The sensitivity of the Millstone 440-MHz radar system is such that coherent echoes from E region irregularities can be observed over a 90-dB dynamic range above the incoherent scatter background. At antenna elevation angles between 4° and 20°, aspect angles between 0° and 10° (from perpendicularity with the magnetic field) are viewed at E region heights at invariant latitudes between 61°Λ and 57°Λ. During disturbed conditions, when convection electric fields in excess of 15 mV/m and E region irregularities span this range of latitudes, antenna scanning experiments have been performed to determine the aspect angle sensitivity with high precision. Our measurements are unique in that they provide a clear high-frequency description of the variation in both power and Doppler shift as functions of aspect angle, all the way from a region where the waves are known to be linearly unstable, in a direction perpendicular to the geomagnetic field, to as much as 10° away from perpendicularity. We find that the 440-MHz aspect sensitivity is about −15 dB deg−1 for aspect angles between 0° and 3°, −10 dB deg−1 for aspect angles between 3° and 6°, and −7 dB deg−1 for aspect angles between 6° and 9°. The magnitude of the phase velocity is at an approximate ion acoustic level (350 m/s) for aspect angles <2° and decreases to <200 m/s as the aspect angle increases to >3°. For highly disturbed conditions the magnitude of the velocity can increase to >700 m/s for aspect angles <2°. The tendency for the altitude of the most intense return to decrease by ∼5 km as the aspect angle increases beyond 2° can be explained as a consequence of the variation of aspect angle with height.
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